Method of forming fluid pressure seal between elements of a container having an interior coating of porcelain enamel



Jan. 15, 1957 R. H. ASHLEY 2,777,194-

METHOD OF FORMING FLUID PRESSURE SEAL BETWEEN ELEMENTS OF A CONTAINERHAVING AN INTERIOR COATING OF PORCELAIN ENAMEL Filed Aug. 9, 1954 2SheetsSheet l INVENTOR. OBERT H ASHLEY,

1957 R. H. ASHLEY 2,777,194

METHOD OF FORMING FLUID PRESSURE SEAL BETWEEN ELEMENTS OF A CONTAINERHAVING AN INTERIOR COATING OF PORCELAIN ENAMEL Filed Aug. 9, 1954 2Sheets-Sheet?! IN VEN TOR. fiecwrf/ A9145 7 Robert H. Ashley, Bryn Mawr,Pa., assignor to John Wood Company, New York,,N. Y., a corporation ofDelaware Application August 9, 1954, Serial No. 448,723

11 Claims. (Cl. 29-458) My invention relates generally to containers,and is particularly applicable to the production of water heater tankswhich are coated on the interior surface with porcelain enamel, and tothe production of other types of procelain enamel coated tanks, etc.

Most of the commercial domestic water heaters manufactured today eitherinclude a steel water tank which after fabrication is completelygalvanized to provide the water tankwith a corrosion resistant coatingon its in terior and exterior surfaces, or include a steel water tankwhich is provided with a corrosion resistant interior coating ofporcelain enamel.

A relatively limited number of water heater tanks are made fromnon-ferrous metals, such as copper, or alloys such as Everdur or Monelmetal, or from steel provided with a corrosion resistant cladding, or asteel tank with a non-ferrous liner. However, the cost of water tanksformed from such non-ferrous metals, clad steels, etc. is a very high ascompared with the cost of a galvanized water tank or a water tankprovided with an interior coating of porcelain enamel. p It is wellknown in the art that one of the difliculties encountered when theinterior surfaces of a steel water tankare coated with an interiorcoating of porcelain enamel after the water tank has been completelyfabricated is that it is impossible practically to properly inspect theinterior procelain enamel coating to determine whether there are anydefects in the coating. If the porcelain enamel coating be defective,when the water heater is put in use, electrolytic corrosion of the steelwill set in where the coating is defective.

In order to overcome the difficulties encountered when the interiorporcelain enamel coating is applied to the steel water tank after thewater tank has been corn-v pletely fabricated, it is now rather commonpractice to weld together the water tank steel head closure and theouter cylindrical steel shell as one subassembly; and the steel bottomclosure and the steel cylindrical flue pipe 'as another subassembly. Thetwo subassemblies, separately, are then provided with procelain enamelcoatings on their surfaces which will be interior surfaces when the twosubassemblies are welded together to form a water tank. The porcelainenamel coatings on such subassemblies may be inspected readily, beforeassembling them together, to determine whether there are any defects inthe porcelain enamel coatings.

Said porcelain enamel coated subassemblies are assembled together bypressing the'subassembly of the bottom closure and the flue tube intothe shell, until a top portion of the flue tube extends through anoutwardly flanged opening in the head closure. The flue tube and the topof the flanged opening are then circumferentially welded together by anyof the well known electric or gas metal welding processes. Thedifliculty encountered is that the necessary welding heat conductedthrough the steel at and about the weld point will partially or whollydestroy the contacting porcelain enamel coatings on the interior of theoutwardly extending flanged opening I Patented Jan. 15, 1957 in the headand on the exterior of the flue tube at, and for a certain distance awayfrom, the point of weld. This is due to the fact that the metal weldingtemperatures for steel are considerably in excess of the melting pointof theporcelain enamel coatings, which usually have a melting pointaround 1550 F. to 1600 F.

The porcelain enamel coated bottom closure also is circumferentiallywelded to the interiorly porcelain enamel coated shell, the weld pointusually being at the lower part of the depending flange of the bottomclosure. The necessary heat of welding conducted through the steel atand about this weld point also will partially or wholly destroy thecontacting porcelain enamel coating on the interior of the shell and onthe exterior of the depending flange of the bottom closure at, and for acertain distance away from, the point of weld. However, the dependingflange of the bottom closure is of sufficient length and diameter thatthe weld heat usually will be dissipated sufliciently as to leave animpervious porcelain enamel coating without defects at the areacontacted by water within the tank.

It is now rather common practice to include an anode of a metal moreactive than steel, such as magnesium, connected to the steel water tankto maintain the steel structure electrically negative to protect fromcorrosion the steel base which might be exposed to the water in the tankbecause of defects in the interior porcelain enamel coating of the watertank.

The anode is suspended from an anode holder screwed into an openingformed in the head closure, and the anode depends therefrom to a pointnear the bottom closure. Such an anode aifords cathodic protection forthe steel at any defective point in the procelain enamel coating whichmight have developed by reason of the metal welding of the flange of thebottom closure to the outer shell, or at any other porcelain enamelcoated portion, except in the vicinity of the area where the flue tubeenters the flanged opening in the head closure. Because it is necessaryto suspend the anode from the head closure, as the anode must be solocated that it can be replaced if and when used up, the anode, as iswell known in the art, affords little or no cathodic protection at anydefective point in the porcelain enamel coating subjected to water atthe junction of the inner side of the flanged opening in the headclosure and the flue tube.

The water heater industry has expended large sums of money and time inresearch in an endeavor to find a commercially practical solution tothis long existing and well known problem of the destruction of theporcelain enamel coatings caused by the metal welding operations. Manysolutions to the problem have been considered and tried with more orless unsatisfactory results, such as the interposing of heat insulatinggaskets between the contacting porcelain enamel surfaces in the vicinityof a point of weld; use of inverted head closures to extend flanges sothat the point of weld would be more remote from the interior porcelainenamel coating of the water tank exposed to water; etc.

Some water heater manufacturers have even located the flue at the lessdesirable position at the outside of the shell of the tank, instead ofpassing the flue tube through the tank, to avoid having to make acircumferential metal weld at the point where the internal flue tubeprojects through a flanged opening in a closure, such weld having to bemade after the parts have been provided with a porcelain enamel coating.

Heretofore, it has been uniformly thought and believed by the industrythat such a metal weld was an absolute necessity to assure that thewater tank be fluid tight when subjected to usual water pressures andpulsations.

One object of my invention is to provide a method of ice forming a fluidpressure tight seal between elements of a container which have coatingsof porcelain enamel thereon and without a metal welding together of suchelements after the porcelain enamel has been applied. 1

Another object of my invention is to provide a method of fusing togetherpreviously coated, dried and fired contacting porcelain enamel surfaceson elements of a container by means of a high frequency electricalcurrent induced at a limited area and for a very limited period of time.

Another object of my invention is to provide a method of heating by ahigh frequency electrical current the contacting porcelain enamelcoatings only to a temperature sufficient to fuse the coatings togetherand to continue to be bonded to the respective steel bases, with thetemperature of the coatings not being permitted to rise to a temperatureas would cause defects in the porcelain enamel coated surface.

Other features of my invention and a complete understanding thereof maybe had by reference to the following detailed description with referenceto the accompanying drawings. 9

' In said drawings:

Fig. 1 is a somewhat diagrammatic elevation view of an assembledporcelain enamel lined water tank ready to be joined together by the useof my improved method.

Fig. 2 is atop plan view of the structure shown in Fig. 1.

Fig. 3 is a fragmentary vertical sectional view, on a larger scale,taken on the line 3--3 in Fig. 2.

Fig. 4 is a fragmentary plan view, on a larger scale, of the headclosure and flue pipe as indicated in Fig. 2, and with apparatus used inpracticing the invention, the induction coil and oscillation generatorbeing shown diagrammatically.

Fig. 5 is a fragmentary sectional view of the structure shown in Fig. 4.

Fig. 6 is an elevation view of an open-ended single turn helical glassring.

Fig. 7 is a somewhat diagrammatic vertical sectional view of a form ofmeans for expanding the top of the flue tube shown in Fig. 3.

Fig. 8 is a fragmentary sectional view, on a larger scale, showing thetwo porcelain enamel coatings fused together to form a fluid pressureseal between the elements and without metal welding.

Referring more particularly to Figs. 1 to 3, inclusive, the water tankindicated generally at 1 includes a cylindical shell body member 2 whichis provided with a top closure member 3. The top closure member 3,conveniently, has a circumferential flange 4 which is adapted to be pushfitted into the top of the shell member 2. The head member 3 and shell 2are integrally welded together, conveniently, by a circumferential weld,as indicated at 5.

The bottom closure member 6 has a circular opening 7 formed therein,conveniently with a circumferential upturned flange 8. The lower end ofthe flue tube 9 is fitted into the flanged opening 7. The flue tube 9,conveniently, is integrally circumferentially welded to the bottommember 6 by a circumferential weld as indicated at 10. The lower end ofthe flue tube 9 is in open communication with the combustion chamber 11formed in the shell 2 below the bottom closure member 6, and in whichcombustion chamber a gas burner (not shown) is located.

The shell member 2 with the head closure 3 integrally welded thereto,forming a subassembly, is then provided on the inner surfaces thereofwith a porcelain enamel coating 12.

Porcelain or vitreous enamel is a glass of a special composition, andwell known as an article of commerce. The porcelain frit is mixed withwater and other materials to a paint-like consistency and applied in aWet state, conveniently by spraying or dipping, to the interior surfacesof the subassembly composed of the head memher 3 and shell member 2.After the frit mixture has been dried, the subassembly is subjected to aheating operation in a furnace in which the shell and head assembly isheated to approximately l550 to 1600 Fahrenheit to fuse the porcelainenamel coating to the head member 3 and shell 2.

i The bottom closure member 6 and the flue tube 9 integrally weldedtogether, forming a subassembly, is also provided on the outer surfacesthereof with a porcelain enamel coating 12 in the same manner asheretofore described with reference to the subassembly comprised of theshell member 2 and head closure 3.

The subassembly comprised of the shell member 2 and head closure 3 andthe subassembly comprised of the bottom closure 6 and the flue tube 9are then assembled together by pressing the bottom closure 6 into theshell member 2 with the flue tube 9 projecting through an opening 15,conveniently having an upturned flange 16, formed in the head member 3.

Thus, the entire interior surface of the tank 1, including the outersurface of the flue tube 9, is provided with a porcelain enamel coatingat all of the interior surface areas of the tank which will be incontact with water, or other fluid, when the tank is filled.

The porcelain enamel coating 12 on the flanged opening 15 in the headmember 3 and the porcelain enamel coating 12 on the outside of the fluetube 9 are in contact with each other throughout the area where the fluetube extends through the head member.

The limited area where the flue tube 9 and flanged opening 15 are incontact with each other is then heated with an induced high frequencycurrent by positioning such limited area within the effective field ofan induction heating coil 20, connected to the output of a suitableelectrical oscillation generator indicated at 21. Sufficient highfrequency energy is then transferred by induction from the oscillationcoil to induce electrical currents within the steel which will heat thecontacting porcelain enamel surfaces 12 at said area to such a degreethat the porcelain enamel coatings will be fused together and provide acontinuous porcelain enamel seal 23 fused to both the metal of the fluetube and the metal of the flanged opening.

As is well known in the art, the size and number of turns of the coilare governed by the frequency of the oscillation generator, with dueconsideration being given to the dimensions of the assembly to be placedwithin its effective field.

Although it would be possible to use a current having a frequency fromabove 60 cycles to upward of 5 megacycles, I have found that it iscommercially practical with presently developed apparatus to use a highfrequency current of the order of kilocycles to 400 kilocycles, and fora period of time between 30 seconds and 60 seconds. Of course, theperiod of time required to fuse the porcelain enamel coatings togetherdepends upon the frequency of the induced electrical current and thethickness of the steel.

I have found that it is preferable, although not essential, to addadditional fusable material at the top of the flange 16 where the fluetube 9 projects therethrough before subjecting the assembly to the highfrequency electrical current. Such additional material may be in theform of porcelain enamel frit from which the previously fired porcelainenamel coatings 12 were formed, or may be in the form of a glass ring,as indicated at 25 in Fig. 6, the glass ring 25 being slipped over theupper end of the flue tube 9 and coming to rest on the upper surface offlange 16. Although a solid glass ring may be used, I have found that itis preferable to use an open-ended single turn helical glass ring.

My improved method produces an interfusion of the porcelain enamelcoatings free from defects at the joinder. The parts are bonded togetherwith a fluid tight j oint therebetween having a tensile strength of theorder of at least 6000 pounds per square inch, and without any metalwelding together of the parts. The tensile strength of the joinder iswell in excess of any pressures to which such water heater tanks aresubjected, and well in excess of the maximum test pressures which suchwater tanks must withstand under the regulations of any of the variousauthorities.

If the flue tube 9 and flanged opening 15 in the head closure should notbe truly circular when formed and porcelain enameled due to commercialmanufacture, it may be found desirable to use an internal expander meansin the flue tube 9 at the location of the induction heating coil 20. Theuse of an internal expander would assure that the porcelain enamelsurfaces 12 will be in contact with each other and fused together freefrom defects at the joinder surfaces when the porcelain enamel coatingsare heated to the melting point by the induction heating coil 20 andfused.

Fig. 7 shows somewhat diagrammatically one of a number of well knowntypes of expander means which may be utilized. An outer supporting ring30 is positioned around the flange 16 of the opening in-the head closure3. An expanding segmental mandrel 31, of a type well known in art, isinserted in the top of the flue tube 9. The mandrel 31 is expanded bymeans of a draw bar 32 which may be actuated by an air cylinder (notshown), operatively connected to the upper end of the draw bar 32. Asthe draw bar 32 is moved upwardly, as in Fig. 7, the segmental mandrel31 is expanded to expand the flue tube 9 to the dregree desired.

The porcelain enamel coatings 12 on the flange of the bottom closure 6and on the cylindrical shell body member 2 at the area where in contactwith each other, as best shown in Fig. 3, may also be heated with aninduced high frequency current, by the same method as heretoforedescribed, to produce an interfusion of said porcelain enamel coatings12 at this area. These parts, correspondingly, will be bonded togetherwith a fluid tight joint therebetween having a tensile strength well inexcess of any required maximum test pressures, and without any metalwelding together of the parts.

I do not desire to limit myself to the specific details of the method asherein set forth, as it is obvious that various modifications may bemade therein without departing from the essential features of myinvention as defined in the appended claims.

I claim:

1. The method of joining, without metal welding, fired porcelain enamelcoated steel elements of a container in fluid pressure tightrelationship comprising assembling said elements with their respectivefired pro-oelain enamel coatings in direct contact at a limited area;subjecting only the immediate vicinity of said limited contact area toan induced high frequency electrical current for a period of timesufficient to raise the temperature of said fired porcelain enamelcoatings at said limited contact area to cause an interfusion of saidporcelain enamel coatings at said limited contact area; and cooling theassemblage to harden the interfused porcelain enamel coatings bonded atopposite sides to the respective steel elements.

2. The method of joining, without metal Welding, fired porcelain enamelcoated steel elements of a container in fluid pressure tightrelationship comprising assembling said elements with their respectivefired porcelain enamel coatings in direct contact at a limited area;subjecting only the immediate vicinity of said limited contact area toan induced high frequency electrical current of the order of 140kilocycles to 400 kilocycles for a period of time suflicient to raisethe temperature of said fired porcelain enamel coatings at said limitedcontact area to cause an interfusion of said porcelain enamel coatingsat said limited contact area; and cooling the assemblage to harden theinterfused porcelain enamel coatings bonded at opposite sides to therespective steel elements.

' 3. The method of joining, without metal welding, fired porcelainenamel coated steel elements of a container in fluid pressure tightrelationship comprising assembling said elements with their respectivefired porcelain enamel coatings in direct contact at a limited area;subjecting only the immediate vicinity of said limited contact area toan induced high frequency electrical current for a period of timesufficient to raise the temperature of said fired porcelain enamelcoatings at said limited contact area only to such a degree as to causean interfusion of said porcelain enamel coatings at said limited contactarea; and cooling the assemblage to harden the interfused porcelainenamel coatings bonded at opposite sides to the respective steelelements.

4. The method of forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue member and said bottom closure together as a secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fuse saidcoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closure, respectively, at oppositeends of said shell and with said flue tube projecting through saidflanged opening in said head closure; subjecting only the immediatevicinity of the limited contact area where the fired porcelain enamelcoating on said flanged opening and the fired porcelain enamel coatingon said flue member are in contact with each other to an induced highfrequency electrical current for a period of time sufficient to raisethe temperature of said fired porcelain enamel coatings at said limitedcontact area to cause an interfusion of said porcelain enamel coatingsat said limited contact area; and cooling the assemblage to harden theinterfused porcelain enamel coatings bonded at opposite sides to therespective steel elements; whereby, a fluid pressure tight seal isformed between the elements without a metal welding together of suchelements after the fired porcelain enamel coatings have been applied.

5. The method of forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue member and said bottom closure together as a secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fuse saidcoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively,

other to an induced high frequency electrical current of the order of140 kilocycles to 400 kilocycles for a period of time suificient toraise the temperature of said fired porcelain enamel coatings at saidlimited contact area to cause an interfusion of said porcelain enamelcoatings at said limited contact area; and cooling the assemblage toharden the interfused porcelain enamel coatings bonded at opposite sidesto the respective steel elements; whereby, a fluid pressure tight sealis formed between the elements without a metal welding together of suchelements after the fired porcelain enamel coatings have been applied.

6. The method of forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue member and said bottom closure together as a secondsubassembly; separately porcelain enamel coattng said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fuse saidcoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively, at oppositeends of said shell and with said flue tube projecting through saidflanged opening in said head closure; subjecting only the immediatevicinity of the limited contact area where the fired porcelain enamelcoating on said flanged opening and the fired porcelain enamel coatingon said flue member are in contact with each other to an induced highfrequency electrical current for a period of time sutficient to raisethe temperature of said fired porcelain enamel coatings at said limitedcontact area only to such a degree as to cause an interfusion of saidfired porcelain enamel coatings at said limited contact area; andcooling the assemblage to harden the interfused porcelain enamelcoatings bonded at opposite sides to the respective steel elements;whereby, a fluid pressure tight seal is formed between the elementswithout a metal welding together of such elements after the firedporcelain enamel coatings have been applied.

7. The method of: forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell memher, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue member and said bottom closure together as a. secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a. water heatertank; separately firing said first and second subassemblies to fuse saidcoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively,

at opposite ends of said shell and with said flue tube projectingthrough said flanged opening in said head closure; applying pressure tomaintain the fired porcelain enamel coatings on said flanged opening andsaid flue member in contact with each other at the area of said flangedopening; subjecting only the immediate vicinity of the limited contactarea where the fired porcelain enamel coating on said fianged opcn andthe fired percelain enamel coating on said flue member are in contactwith each other to an induced high frequency electrical current for aperiod of time suflicient to raise the ternperature of said firedporcelain enamel coatings to cause an interfusion of said porcelainenamel coatings at said limited area at said limited contact area; andcooling the assemblage to harden the interfused porcelain enamelcoatings bonded at opposite sides to the respective steel elements;whereby, a fluid pressure tight seal is formed between the elementswithout a metal welding together of such elements after the firedporcelain enamel coatings have been applied.

8. The method of forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue mem her and said bottom closure together as a secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fuse saidcoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively, at oppositeends of said shell and with said flue tube projecting through saidflanged opening in said head closure; expanding said flue member at thearea of said flue opening to maintain the porcelain enamel coating onsaid flue member in contact with the porcelain enamel coating on thearea of said flanged opening; subjecting only the immediate vicinity ofthe limited contact area where the tired porcelain enamel coating onsaid flanged opening and the fired porcelain enamel coating on said fluemember are in contact with each other to an induced high frequencyelectrical current for a period of time sutficient to raise thetemperature of said fired porcelain enamel coatings at said limitedcontact area to cause an interfusion of said fired porcelain enamelcoatings at said limited contact area; and cooling the assemblage toharden the interfused porcelain enamel coatings bonded at opposite sidesto the respective steel elements; whereby, a fluid pressure tight sealis formed between the elements without a metal welding together of suchelements after the fired porcelain enamel coatings have been applied.

9. The method of forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue member and said bottom closure together as a secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fusesaidcoatings; cooling said subassemblies to harden their respectivefired porcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively, at oppositeends of said shell and with said flue tube projecting through saidflanged opening in said head closure; subjecting only the immediatevicinity of the limited contact area where the fired porcelain enamelcoating on said flanged opening and the fired porcelain enamel coatingon ,said flue memmeme;

her are in contact with each other to an induced high frequencyelectrical current for a period of time sufficient to raise thetemperature of said fired porcelain enamel coatings at said limited areato cause an interfusi'on of said fired porcelain enamel coatings at saidcontact area; subjecting only the immediate vicinity of the limited areawhere the fired porcelain enamel coating on said shell member and thefired porcelain enamel coating on said bottom closure are in contactwith each other to an induced high frequency electrical current for aperiod of time sufficient to raise the temperature of said firedporcelain enamel coatings at said limited area to cause an interfusionof said fired porcelain enamel coatings at said limited contact area;whereby, a fluid pressure tight seal is formed between all the elementswithout a metal welding together of such elements after the firedporcelain enamel coatings have been applied.

10. The method of forming porcelain enamel lined water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said closure together as a first subassembly; weldingsaid flue memher and said bottom closure together as a secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fuse saidcoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively, at oppositeends of said shell and with said flue tube projecting through saidflanged opening in said head closure; subjecting only the immediatevicinity of the limited contact area where the fired porcelain enamelcoating on said flanged opening and the fired porcelain enamel coatingon said flue member are in contact with each other to an induced highfrequency electrical current of the order of 140 kilocycles to 400kilocycles for a period of time suflicient to raise the temperature ofsaid fired porcelain enamel coatings at said limited area to cause aninterfusion of said fired porcelain enamel coatings at said limited contact area; subjecting only the immediate vicinity of the limitedareawhere the fired porcelain enamel coating on said shell member and thefired porcelain enamel coating on said bottom closure are in contactwith each other to an induced high frequency electrical current of theorder of 140 kilocycles to 400 kilocycles for a period of timesufiicient to raise the temperature of said fired porcelain enamelcoatings at said limited area to cause an interfusion of said firedporcelain enamel coatings at said limited contact area; whereby, a fluidpressure tight seal is formed between all the elements Without a metalWelding together of such elements after the fired porcelain enamelcoatings have been applied.

11. The method of forming porcelain enamel lined Water heater tankscomposed of a cylindrical steel shell member, head and bottom closures,said head closure having an outwardly flanged circular opening, and acylindrical tubular flue member extending longitudinally within saidshell member and connecting said end closures, comprising welding saidshell member and said head closure together as a first subassembly;welding said flue member and said bottom closure together as a secondsubassembly; separately porcelain enamel coating said first and secondsubassemblies on their surfaces, including the interior surface of saidflanged opening, which will be interior surfaces when said subassembliesare assembled together after porcelain enameling to form a water heatertank; separately firing said first and second subassemblies to fusecoatings; cooling said subassemblies to harden their respective firedporcelain enamel coatings; assembling said first and secondsubassemblies together with said end closures, respectively, at oppositeends of said shell and with said flue tube projecting through saidflanged opening in said head closure; subjecting only the immediatevicinity of the limited contact area where the fired porcelain enamelcoating on said flanged opening and the fired porcelain enamel coatingon said flue member are in contact with each other to an induced highfrequency electrical current of the order of kilocycles to 400kilocycles for a period of time sufiicient to raise the temperature ofsaid fired porcelain enamel coatings at said limited area only to such adegree as to cause an interfusi on of said fired porcelain enamelcoatings at said limited contact area; subjecting only the immediatevicinity of the limited area where the fired porcelain enamel coating onsaid shell member and the fired porcelain enamel coating on said bottomclosure are in contact with each other to an induced high frequencyelectrical current of the order of 140 kilocycles to 400 kilocycles fora period of time sufiicient to raise the temperature of said firedporcelain enamel coatings at said limited area only to such a degree asto cause an interfusion of said fired porcelain enamel coatings at saidlimited contact area; whereby, a fluid pressure tight seal is formedbetween all the elements without a metal welding together of suchelements after the fired porcelain enamel coatings have been applied.

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